The invention relates to a process for compressing by hammering a steam generator tube set in a tube plate for the purpose of limiting corrosion under stress.
Steam generators of pressurized-water nuclear reactors comprise a very thick tube plate in which the tubes of the nest of tubes of the generator are set at each of their ends. The tubes are flush with one of the sides of the tube plate which comes in contact with the primary fluid during the operation of the steam generator and project from the other side of the tube plate and communicate with the interior of the body of the steam generator receiving the water to be vaporized.
The setting or expansion of the tubes is carried out by introducing a tool, termed a tool-expander, inside the tube for rolling its wall inside its cavity in the tube plate. This rolling of the tube is carried out from the end thereof which is flush with the first side of the tube plate up to a zone located substantially in the vicinity of the second side of the tube plate. This zone of the tube located in the vicinity of the outlet side of the tube plate therefore constitutes the zone of separation between the part of the tube which is deformed by rolling in the corresponding bore of the tube plate and the undeformed part of the tube. This zone is termed the transition zone. In the transition zone, the wall of the tube is the center of considerable residual tensile stresses which reduce the resistance of the tube to corrosion both on its outer surface in contact with the water to be vaporized and on its inner surface in contact with the primary fluid.
Indeed, there is found, in steam generators of nuclear reactors,after a certain period of operation, a deterioration of certain tubes of the nest in the region where they extend out of the tube plate, i.e., in the vicinity of their transition zones. Destruction by corrosion is in the form of cracking or even holesin the wall of the tube. More or less serious deteriorations on the tubes in zones other than the transition zone have also been found in steam generators, after a certain period of operation. The origin of these deteriorations may be attributed in some cases to the presence of residual stresses, in particular in the inner skin of the tubes.
It has been proposed in French Patent No. 77 13 196, owned by the assignee of the present invention, to effect mechanical destressing of the tubes of steam generators after their expansion in the tube plate. This de-stressing is achieved by means of a tube-expander of special design which enables a slight diametrical expansion of the tube in its transition zone to be achieved. This operation results in a reduction of the stresses in the wall of the tube in the vicinity of its outer surface which comes in contact with the water to be vaporized. In this way corrosion due to the supply water of the steam generator in the vicinity of the tube plate is reduced.
However, this mechanical de-stressing operation by diametrical expansion with a tube-expander does not permit a reduction in the stresses in the wall of the tube in the vicinity of its inner surface, or stresses in the inner skin of the tube. Corrosion by the primary fluid, consisting of water under pressure including boric acid and various conditioning bases, therefore remains considerable in the inner skin of the tube in the transition zone.
There has also been proposed a process for compressing by hammering of the inner surface of the tube in the transition zone. This hammering of the tube, carried out by rotating at very high speed inside the tube a flexible band carrying balls of small size and composed of a hard material, enables the resistance of the tube to corrosion by the primary fluid to be increased. However, in the event of breakage of a ball of hard material, the rubbing of this ball, which has sharp edges, against the inner wall of the tube in the course of the rotation of the flexible band, produces grooves on the inner wall of the tube. These grooves facilitate the corrosion of the inner skin of the tube by the primary fluid.
Methods are also known for achieving internal hardening of tubes such as cast steel tubes, which comprise projecting balls of hard material onto the inner surface of these cast tubes. However, this method has never been used for compressing the inner skin o f expanded tubes of small diameter, such as generator tubes of a pressurized water nuclear reactor, whose inside diameter is a little less than 0.020 m. The operating procedure involved in the interior hardening of cast steel tubes, is obviously not applicable in the case of expanded tubes. It is also quite clear that the devices employed for the internal hardening of the cast tubes are not applicable in the case of tubes expanded in the tube plate of a steam generator, and more particularly in the case where the operations are carried out on a pressurized water nuclear reactor steam generator, after it has been put into service, since the tubes and the water-box of the steam generator are then more or less irradiated.
In this case, the presence of an operator in the vicinity of the irradiated zone must be avoided as far as possible. Consequently, the whole of the tooling of the device must be maintained on the tube plate, and the seal therebetween and the latter must be ensured and the movements of translation of the nozzle in the tube must be controlled automatically, and these various stages of the operation must be supervised without intervening in the steam generator.